The growing prevalence of obesity and associated diseases, such as type II diabetes is a major health concern, particularly among children. Recent evidence has shown that obesity and related diseases might be a consequence of alterations in the developmental processes of a variety of systems involved in energy balance regulation, including the hypothalamic melanocortin system. This neural system, which includes neurons that produce pro-opiomelanocortin (POMC)-derived peptides, is a major negative regulator of energy balance. POMC neurons begin to acquire their unique properties during embryonic life. Notably, cell lineage experiments have shown that a subpopulation of embryonic Pomc-expressing precursors subsequently adopts a NPY phenotype (an orexigenic peptide also expressed in ARH neurons) in adult mice. However, the molecular mechanisms underlying the differentiation and cell fate specification of POMC neurons during development remain largely unknown. MicroRNAs (miRNAs) have recently emerged as critical regulators of brain development. These non-coding small endogenous RNA molecules have important functions in gene regulation and abnormal miRNA maturation impairs neuronal differentiation, induces cell death, and disrupts axon growth. The overall hypothesis of this proposal is that miRNAs are required for the normal development of POMC neurons and that the loss of specific miRNAs during critical fetal periods results in the abnormal maturation of POMC neurons and long-term metabolic dysregulation. Neuroanatomical, neurophysiological, physiological, and molecular approaches will be used to test this hypothesis by addressing the following specific aims. Specific Aim 1. We will evaluate the importance of miRNA maturation in cell fate decision in an immature population of POMC neurons. To this aim, we will generate mouse models in which Dicer (an essential enzyme for miRNA maturation) is deleted in POMC neurons. We hypothesize is that Pomc-expressing progenitors might acquire a non-POMC phenotype (for example a NPY phenotype) in the absence of sufficient miRNA maturation. We will also examine the impact of Dicer deletion on the connectivity of POMC and Pomc ? NPY neurons. Specific Aim 2. We will use an in vivo gene silencing approach to characterize the role of miR-103 and miR-107, specifically, in the timely maturation of POMC neurons during embryogenesis. We will also explore the importance of embryonic miR-103 and miR-107 expression in POMC neurons on POMC neuronal activity and connectivity and on lifelong energy balance regulation and glucose homeostasis. Specific Aim 3. Moreover, we will examine the role of miR103/107 in the metabolic and neuroanatomical defects observed in offspring of obese dams. Together, this work will define a critical role for specific miRNAs in the development of neural systems that are involved in lifelong energy balance regulation. These studies might also provide new insight into the molecular mechanisms by which alterations of the prenatal nutritional environment lead to obesity and diabetes in the offspring, thereby generating new therapeutic opportunities.